1. Field of the Invention
The present invention relates to an alternator in which the generated voltage contains a low porportion of harmonics.
The invention is primarily but not exclusively directed to single-phase alternators in which the inductor (field core) has one pair of poles.
2. Description of the Prior Art
As is already known, the armature reaction field produced within the armature of an alternator as a result of the current which flows therein has a positive sequence component and a negative sequence component with respect to the inductor, the frequency of the negative sequence component being double the frequency of the current which flows within the armature.
Saturation of the pole shoes as a result of the positive sequence component of the armature reaction field causes deformation of the magnetic flux distribution within the air-gap. This has the contributory effect of inducing in the armature windings an electromotive force which contains a high proportion of third harmonics.
Moreover, in single-phase alternators, the negative sequence component of the armature reaction field induces in the field windings of the inductor an electromotive force and a current having a frequency which is double that of the current within the armature. This has the effect of generating a so-called "reflected" field which gives rise to additional iron losses within the magnetic circuit of the inductor, thereby increasing heat build-up during operation of the alternator. This reflected field also induces in the stator an electromotive force whose frequency is three times that of the current which flows within the armature, thus further increasing the proportion of third harmonics. In point of fact, this third-harmonic content has a particularly unfavorable effect on the operation of certain types of supplied equipment.
In order to reduce the proportion of third harmonics in the electromotive force of the armature, certain manufacturers propose to wind the armature of the alternator with a special winding having a reduced pitch. However, this solution is costly and results in the construction of so-called special machines which, in addition, have to be declassified or declared obsolete in most instances.
Accordingly, the aim of the invention is to propose an alternator in which the third-harmonic content of the output voltage is reduced to a minimum, independently of the type and nature of the armature winding.